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My friend Ian Nisbet has a couple of photos that tell a revealing biological story. One shows him, a vigorous young fellow with a shock of curly black hair, holding a sleek looking common tern. The other shows him more than 20 years later, a grizzled veteran with a white beard and snowy locks, holding a sleek looking common tern. The kicker is that it's the same man and the same bird-the latter appearing not a whit the worse for wear.

"Common terns die before they grow old," Nisbet likes to say, using a bit of poetic (or gerontological) license. He is a seabird biologist who spent 30 years studying and banding terns on islands off Cape Cod. Among the more interesting facts he uncovered were those on the performance of a group of terns aged 18-23 years.

"The median age for terns is about ten years, but there is no visible difference between very old terns and younger ones," he told me. "And there is little sign of decline on performance, function, or their survival rate."

In fact, Nisbet's "very old terns" out-competed all of the other age groups in the nesting colony. (In one breeding season, he found only 64 of the 3,548 banded birds on the island were 18 years or older.) They laid their eggs much earlier, produced larger clutches, and fledged more healthy offspring. Nisbet didn't discover any of the old birds dead on the island.

What's going on there? Nisbet has been one of the scientists looking into the phenomenon of "slow aging" in animals --the possibility that some grow old so slowly (like very ancient trees) that we aren't able to detect changes in their appearance or functioning. Birds seem to do better than mammals of similar size, despite their high metabolic rate and blood glucose, whether in the wild or in zoos. A three-year-old mouse is ready to turn up its toes, whereas a similar-sized sparrow may live to be 15 or more.

One intriguing possibility is the capacity of cells and tissues to withstand deterioration. As animals eat, the metabolic processes that build animal cells and generate energy also create oxidizing agents that attack cells. Research has already shown that some birds produce enzymes and other organic substances called antioxidants that counter such effects. Also, delayed maturity seems to be correlated with vigorous old age. Seabirds such as terns and petrels put off maturity, starting to breed at three or four years.

Further research will demonstrate that even Ian Nisbet's terns are subject at some point to aging. "Seabirds live longer than the working lifespan of biologists," he told me. "We don't know what terns die of. They die mostly in winter, and away at sea."